A material's magnetic properties pertain generally to how the material behaves when exposed to magnetic fields. There are several commonly recognized types of magnetic material including diamagnetic, paramagnetic, ferromagnetic, antiferromagnetic, ferromagnetic, and superparamagnetic. The main characteristics of each type are overviewed in Engineering Electromagnetics (Hayt, Jr., William H., pg 306-310) and are described below for the three most common types.
Diamagnetic materials have atoms which have no permanent magnetic moments. Specifically, the electron spins and orbital motions balance out within each atom such that the net moment of each atom is zero. When a diamagnetic material is exposed to an external magnetic field, the external magnetic field induces magnetic moments in each atom which are directed opposite to the external magnetic field. This alignment of atomic moments decreases the magnitude of the internal magnetic field within the material below the magnitude of the applied field.
Paramagnetic materials have atoms which each have a small magnetic moment, but the random orientation of the atoms within the material produces an average magnetic moment of zero. When an external field is applied, the moment of each atom tends to align with the external field. This alignment of atomic moments increases the magnitude of the magnetic field within the material above the magnitude of the applied external field.
In ferromagnetic materials each atom has a relatively large dipole moment caused primarily by uncompensated electron spin moments of electrons, for example, in the d and f shells. Interatomic forces cause these moments to line up in a parallel fashion over regions called domains. Prior to applying an external field, each domain will have a strong magnetic moment. However, due to cancellation of domain moments, which vary in direction, the material as a whole has no magnetic moment. Upon applying an external magnetic field, the domains with moments in the direction of the external field get larger while the other domains get smaller and, therefore, the magnitude of the magnetic field within the ferromagnetic material gets much larger than the magnitude of the applied external field. Furthermore, upon removing the external magnetic field a residual dipole field remains in the material. Each ferromagnetic material is characterized by a hysteresis loop which represents the relationship between B, the magnization of the material, and H, the applied external field.
The magnetic properties of a material can greatly affect the utility of the material. Accordingly, the utility of materials can be greatly extended by changing their magnetic properties. Well known uses of magnetic materials include transformers, electric motors, electromagnets, micromachine parts, and magnetic tags. For example, micromachines, which incorporate the movement of micron-scale parts, currently must be made from iron compounds because these materials have the necessary magnetic characteristics. It would be highly advantageous to have other materials having the necessary magnetic properties for use in micromachines or other applications where magnetic materials are needed. In particular, it would be advantageous to have a magnetic material which is similar and integrated with the substrate material upon which it is situated.
One material which forms the basis for many high technology applications is silicon. There are a variety of forms of silicon which are used in various applications. For example, porous silicon, which can be made by, for example anodic etching, can be used for applications requiring photoluminescing. Another form of silicon is known as amorphous silicon. Silicon oxides (SiO.sub.x) are also important materials in many applications.
One form of silicon which has been recently described is spark-processed silicon (sp-Si). Spark processing, which is described in U.S. Pat. No. 5,397,429 creates a silicon oxide material. This silicon oxide material which is distinct from porous silicon, is known to photoluminesce. U.S. Pat. No. 5,397,429 does not disclose or suggest that spark processing of silicon has any effect on the magnetic properties of that material.
Natural silicon is a diamagnetic material. Porous silicon is thought to be weakly ferromagnetic as is amorphous silicon.
The ability to efficiently modulate the magnetic properties of silicon materials and other materials would be highly advantageous and would make it possible to significantly extend the useful properties of these materials.